1. Field of the Invention
The present invention relates to a gated proportional valve, particularly a gated diaphragm flow control valve used in dispensing fuel in a proportional blending device.
2. Description of the Related Art
Gated diaphragm proportional flow control valves are known in the art of fuel dispensing, automotive and industrial control. FIG. 1 shows a schematic representation of a prior art gated diaphragm flow control valve 1, which has been used in the above applications and particularly in a fuel dispenser with proportional blending. Valve 1 includes an inlet 2 and an outlet 3 for, respectively, receiving and discharging a flow of fluid such as fuel. Inlet 2 leads to an inlet path 4, and outlet path 5 leads to outlet 3. Inlet path 4 includes two fluid flow openings, valve inlet opening 6 and gate inlet opening 7. Gate inlet opening 7 is opened and closed by a gate inlet valve 10, which is reciprocated from opened to closed positions by a gate inlet solenoid 11. Outlet path 5 includes two fluid flow openings, valve outlet opening 8 and gate outlet opening 9. Gate outlet opening 9 is opened and closed by a gate outlet valve 12, which is reciprocated from opened to closed positions by a gate outlet solenoid 13.
Valve 1 body contains a valve opening 14 which is opened and closed by a valve stem 15. Valve stem projects into a gate volume 16, and includes a valve stem diaphragm 17, which seals against walls 18 of gate volume 16. A relief port 20 connects the portion of gate volume 16 below valve stem diaphragm 17 to a valve flow area 21.
In operation of the device of FIG. 1, pressurized fluid is supplied to inlet 2 from a pressurized fluid source. When it is desired to commence fluid flow through valve 1, signals are sent to solenoids 11 and 13 to close gate outlet valve 12 and open gate inlet valve 10, thereby allowing pressurized fluid from inlet 2 and inlet path 4 to pass into gate volume 16 through gate inlet opening 7 but not to leave gate volume 16 through gate outlet opening 9. The operation of solenoids 11 and 13 are coupled, so that when one is in an open position, the other is in a closed position. If there is a sufficient pressure difference between the fluid pressure at inlet 2 and outlet 3, pressure on valve stem diaphragm 17 will move valve stem 15 downwardly, opening valve opening 14 and allowing fluid to flow from inlet 2 to outlet 3. The degree to which valve opening 14 is opened depends upon the proportional value of the inlet pressure to the outlet pressure, such that the valve opening 14 is opened more with a higher pressure difference, and opened less with a lower pressure difference. As a result, the fluid flow through valve opening 14 is proportional to the relative difference between the inlet 2 and outlet 3 pressures.
When it is desired to stop flow through valve 1, signals are sent to solenoids 11 and 13 to simultaneously open gate outlet valve 12 and close gate inlet valve 10, thereby allowing pressurized fluid from gate volume 16 to exhaust through outlet path 5 and outlet 3, while simultaneously preventing fluid from inlet 2 and inlet path 4 from entering gate volume 16. As a result, the pressure on diaphragm 17 is relieved, thereby causing valve stem 15 to move upwardly to close valve opening 14, under the action of pressure on valve stem 15 and diaphragm 17--through relief port 20--from outlet 3. This position of solenoids 11 and 13 prevents fluid from flowing from inlet 2 to outlet 3.
The above-described prior art device operates adequately to provide proportional flow control when there is a sufficiently large difference in the inlet and outlet pressures for the valve 1. However, when it is desired to operate the valve 1 at low flow rates--for example, when a fuel dispenser dispenses a preset volume or dollar amount of fuel and must dispense fuel at a low flow rate as it reaches the preset amount, or when the dispensing nozzle is variably closed or only partially open--the above-described valve can have a tendency to either not open at all or to "flutter" between opened and closed positions in an unstable manner. Accordingly, prior art fuel dispensers which use the valve shown in FIG. 1 generally include an additional valve which controls the flow of fuel at low fuel flow rates. Adding an extra valve to the fuel dispenser, however, can increase costs of the overall system. Adding the extra valve also requires extra controls for controlling the dispensing apparatus to switch between the two valves, increasing costs and adding complexity to the dispensing apparatus.
Valves of the type described above are commonly used in automotive and industrial applications to control fluid flow. As described above, the normal operation of gated diaphragm control valves is to use upstream and downstream pressure to act on a diaphragm and/or a diaphragm stem valve in order to control flow. Gated diaphragm valves are low cost because of their simple construction and are often preferred over more expensive valve technologies. Gated diaphragm valves can be used as normal on/off valves and can perform virtually all of the functions required by a proportional control valve, except reliable low flow operation. Low flow operation of a valve is normally required in precision fluid applications for dispensing exact or very precise volumes. Direct control proportional valves or stepper motor valves perform precision low flow dispensing, but are expensive. Gated diaphragm control valves are inexpensive but perform precision low flow dispensing very poorly. Gated diaphragm valves need a sufficiently large pressure differential across them to work well, and this is not available during low flow conditions.